Transferred electron devices

ABSTRACT

A transferred electron device, such as an oscillator, comprises a body of semiconductor material exhibiting the transferred electron effect, a cathode on the body comprising a region of metal giving good ohmic contact with the body and an anode on the body giving efficient extraction of current carriers and consisting of material other than a metal which would give good ohmic contact. The anode may be an n region of semiconductor or a Schottky barrier diode. The structure is suited to higher frequency operation with larger physical device dimensions.

United States Patent Colliver et a1.

[ 51 June 27, 1972 [54] TRANSFERRED ELECTRON DEVICES [7 2] Inventors: David John Colliver; Cyril Hilsum, both of Malvem, England Minister of Technology in Her Britanic Majesty's Government of the United Kingdom of Great Britain and Norther Ireland, London, England [22] Filed: June9, 1970 [21] Appl.No.: 44,850

[73] Assignee:

12/1970 Tijburg et al. ..317/234 OTHER PUBLICATIONS IEEE Trans. on Elec. Devices, Evaluation 'of Metal-Semiconductors Contacts in Bulk GaAs Oscillation by the Photovoltaic Effect, by Hayashi et al., pp. 200- 201, Jan. 1966 Journal of Applied Physics, Efiect of Nonunifonn Conductivity on the Behavior of Gunn Effect Samples" by Hasty et al., Sept. 1968 pp. 4623- 4632.

Primary Eraminer-Jerry D. Craig Attomey-Hall, Pollock & Vande Sande ABSTRACT A transferred electron device, such as an oscillator, comprises a body of semiconductor material exhibiting the transferred electron effect, a cathode on the body comprising a region of metal giving good ohmic contact with the body and an anode on the body giving efficient extraction of current carriers and consisting of material other than a metal which would give good ohmic contact. The anode may be an 21* region of semiconductor or a Schottky barrier diode. The structure is suited to higher frequency operation with larger physical device dimensions.

3 Claims, 10 Drawing Figures TRANSFERRED ELECTRON DEVICFS The present invention relates to transferred electron devices.

The well-known transferred electron effect is the effect by which electrons in an appropriately doped piece of semiconductor, such as cadmium telluride, gallium arsenide or indium phosphide, are transferred from a conduction band region of high mobility to one of higher energy and lower mobility on the application of an appropriate electric field strength. Devices using the transferred electron efi'ect are known as transferred electron devices.

One well-known consequential effect of the transferred electron efiect is the Gunn effect. This constitutes the formation of domains of high electric field strength which move through the semiconductor with a frequency characteristic of the length of sample taken.

Transferred electron oscillator devices are known to have use in the generation of microwaves. Unfortunately, the smaller devices are severely limited in power output. In order to provide a high frequency device with a reasonable power output it is necessary to find a way of producing a high frequency from a device with larger physical dimensions than those of conventional devices.

\ According to the present invention there is provided a transferred electron device including a body of semiconductor material exhibiting the transferred electron effect, a cathode including a metal giving a good ohmic contact with the body of semiconductor material and an anode giving efficient extraction of current carriers and consisting of material other than a metal which gives good ohmic contact with the semiconductor material.

The anode may include a Schottky barrier diode or heavily doped semiconductor material.

The cathode may include in addition to the metal giving good ohmic contact a region of heavily doped semiconductor material.

An embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 and FIG. 2 are cross-sectional diagrams of known transferred electron devices;

FIG. 3 is a cross-sectional diagram of a transferred electro device embodying the invention;

FIG. 4, FIG. 5, FIG. 6 and FIG. 7 are plan views of transferred electron devices embodying the invention; and

FIG. 8, FIG. 9 and FIG. 10 are cross-sectional diagrams of alternative transferred electron devices embodying the invention.

Hitherto, conventional transferred electron devices have been of two types. The first type uses an epitaxial layer of high purity gallium arsenide, known as the n layer, typically about 1 ohm centimeter in resistivity, deposited on a substrate of highly conducting gallium arsenide, whose resistivity is about 0.01 ohm centimeter or less. This may be called the longitudinal structure. One contact is made on the bottom side of the substrate, and the other on the top surface of the epitaxial layer. This second contact can be made of metal or it may be in the form of a second epitaxial layer deposited on top of the first, this second layer (the n layer) being doped heavily with a donor impurity, and therefore having low resistivity (i.e. being highly conductive).

The second type of structure which has been suggested for a transferred electron device is shown in FIG. 1 and FIG. 2. In this type of transferred electron device the high purity epitaxial layer 1 is deposited on a substrate 3 of the special form of gallium arsenide known as semi-insulating gallium arsenide. The substrate 3 is therefore electrically inert and acts purely as a mechanical support and a seed during the deposition process. This is an example of the structure known as the transverse structure, and requires two contacts to be made to the epitaxial layer 1. In other words, the transverse structure includes having contacts made to the same surface.

Hitherto all proposals for making transferred electron devices with the transverse structure have had these two contacts made from the same material. For example both contacts may be made from a metal, such as silver-tin alloy, silver-indium-germanium alloy or gold-nickel-germanium alloy, or from highly doped epitaxially deposited gallium arsenide, deposited on the surface or in prepared depressions made in the surface. Any of these techniques can readily be performed by the con ventional methods of photoengraving as used in integrated circuit technology. Such contacts are shown in FIG. 1, where two contacts 5, 7 are deposited on the epitaxial layer 1.

In the second type of contact the doping is with a donor impurity such as sulphur and can be done during deposition of the contact. In theory the contact areas could be doped by local diffusion of the donor impurity, but in practice this is not often done since it is more difiicult to maintain the properties of the operative part of the layer during the diffusion. Such contacts are shown in FIG. 2, where two contacts 9, 11 are made during deposition of the expitaxial layer 1.

However, the invention relies for its efiect upon the electrodes of a transferred electron oscillator being of different materials.

Such a structure has not hitherto been considered because it involves two contacting technologies.

However, transferred electron devices both of whose electrodes are made of n -doped gallium arsenide have not been made to work efi'rciently and transferred electron devices both of whose electrodes are made of metals giving good ohmic contact do not work satisfactorily because metals giving good ohmic contact with transferred electron efi'ect semiconductors such as gallium arsenide (such as tin, indium or alloys of them) are low melting point metals and are prone to breakdown under the high field and high current density conditions of the anode. However, it is only at the cathode that good ohmic contact is necessary and this type of breakdown does not occur at the cathode. What is required for the anode is an electrode that extracts carriers efliciently. A transferred electron device having a good ohmic contact at the cathode and efficient extraction at the anode is shown in FIG. 3.

In FIG. 3 the epitaxial layer 1 of n-type gallium arsenide is, as before, grown on the substrate 3 of semi-insulating gallium arsenide. An anode 13 is made from n -doped gallium arsenide deposited in a prepared depression in the surface of the layer 1. Such an electrode is an efficient extractor of current carriers. A cathode consists of a region 15 of n doped gallium arsenide deposited in a similar way to that of the anode but with a layer 17 of metal deposited partly on the region 15 and partly on the epitaxial layer 1 between the region 15 and the anode 13. The metal in the layer 17 must be such as to give a good ohrnic contact with the gallium arsenide; for example tin, indium or any alloy containing tin or indium or both. A voltage source V is connected between the region 15 and the anode 13.

The anode l3 and the region 15 may be made by local epitaxy in depressions formed by etching the surface of the epitaxial layer 1. The depressions may have a depth less than, as great as or more than the thickness of the epitaxial layer 1. Alternatively the anode 13 and the region 15 may be made by local diffusion of donor impurity.

The shape and area of the layer 17 may take any of many alternative forms and consequently the invention is applicable to devices having annular electrodes and devices which have additional electrodes between the anode and the cathode.

FIG. 4, FIG. 5, FIG. 6 and FIG. 7 are alternative plan views of transferred electron oscillators showing alternative shapes and areas of the layer 17. In FIG. 4 a region 15a corresponding to the region 15 in FIG. 3 is rectangular and parallel to the anode 13. A layer 17a corresponding to the layer 17 in FIG. 3 is rectangular and overlaps the region 15a and the space between the region 15a and the anode 13.

In FIG. 5 a layer 17b corresponding to the layer 17 in FIG. 3 is triangular with a single apex overlapping the space between the region 15a and the anode l3.

In FIG. 6 a region b corresponding to the region 15 in FIG. 3 is in the shape of a parallelogram set at an angle to the anode 13. A layer 17c corresponding to the layer 17 in FIG. 3 is also a parallelogram having sides parallel to the region 15b. The layer 17c overlaps the region 15b and the trapezoidal space between the region 15b and the anode l3.

FIG. 7, like FIG. 6, has the region 15b in the shape of a parallelogram set at an angle to the anode 13. A layer 17d corresponding to the layer 17 in FIG. 3 is triangular with a single apex overlapping the space between the region. 15b and the anode 13 at their closest point.

FIG. 8 is a cross-sectional diagram of part of a transferred electron oscillator. The epitaxial layer 1 of n-type gallium arsenide grown on the substrate 3 of semi-insulating gallium arsenide has in its upper surface a plurality of depressions. An anode 13 is made from n -doped gallium arsenide deposited in a depression at one end of the device and a plurality of further electrodes 15,, 15 15 is made from n"'-doped layer arsenide deposited in the other depressions. A plurality of layers 17,, 17,,..., made of metal giving a good ohmic contact with the gallium arsenide is deposited on the surface of the layer 17 overlapping the electrode 15 and the space between the electrode 15 and the electrode 15 to form the main cathode area, the layer 17 overlapping the electrode 15 and the space between the electrode 15 and the electrode 15 and so on. By this means each layer such as 17 forms a transferred electron oscillator with an electrode such as 15 which therefore acts as an anode and is connected to the next cathode such as 17 and the resulting device constitutes a series chain of transferred electron oscillators which may be energized by a single voltage source V connected between the electrode 15 and the anode 13.

Clearly the voltage source V may instead be connected between the cathode 17 and the anode 13; in that case the electrode 15 may be omitted, as shown in FIG. 9.

FIG. 10 is a cross-sectional diagram of an alternative transferred electron oscillator. The epitaxial layer 1 of n-type gallium arsenide grown on the substrate 3 of semi-insulating gallium arsenide has deposited on it, as before, the cathode 17 made of a metal giving a good ohmic contact with the gallium arsenide. In addition an electrode 19 forming a conventional Schottky barrier diode with the layer 1 constitutes the anode. For example, the anode 19 may be made of nickel so deposited as to form a Schottky barrier diode with the layer 1.

It is well known that a Schottky barrier is an efiicient extractor of current carriers, and therefore the device illustrated in FIG. 10 will act as a transferred electron oscillator when a voltage source V is connected between the cathode 17 and the anode 19.

We claim:

1. A transferred electron device of the transverse structure type comprising a semi-insulating substrate of semiconductor material selected from the group of materials exhibiting the transferred electron effect, an epitaxial layer of n-type semiconductor material deposited on a surface of said sub strate, the material of said epitaxial layer being selected from said group of materials; an anode and a cathode formed in spaced relationship to one another on said epitaxial layer, said cathode comprising a region of high conductivity semiconductor material and a low ohmic metal electrode disposed on the surface of said epitaxial layer between said anode and cathode such that said metal electrode overlaps the juncture between said region of high conductivity semiconductor material and said epitaxial layer, said anode being fabricated of a materialdifi'erent from the metal of said metal electrode and having poorer ohmic contact to said epitaxial layer than is exhibited by said metal electrode, said anode being operative to effect efficient extraction of current carriers from said epitaxial layer and consisting of a material selected from the group of materials consisting ofhighly conductive semiconductor material and material of the type forming a Schottky barrier with said epitaxial layer.

2. A transferred electron device as claimed in claim 1 comprising a further electrode disposed on the surface of said epitaxial layer between said anode and said cathode, said further electrode comprising a further region of metal giving good ohmic contact with said epitaxial layer and a highly conductive semiconductor region adjacent a part of said further region of metal.

3. A transferred electron device as claimed in claim 1 and comprising a plurality of further electrodes disposed in spaced relation to one another on the surface of said epitaxial layer between, and in spaced relation to, said anode and said cathode, each of said further electrodes comprising a region of high conductivity semiconductor material and a low ohmic metal layer disposed on the surface of said epitaxial layer in overlapping relation to the juncture between its associated region of high conductivity semiconductor material and said epitaxial layer.

l i il 

1. A transferred electron device of the transverse structure type comprising a semi-insulating substrate of semiconductor material selected from the group of materials exhibiting the transferred electron effect, an epitaxial layer of n-type semiconductor material deposited on a surface of said substrate, the material of said epitaxial layer being selected from said group of materials; an anode and a cathode formed in spaced relationship to one another on said epitaxial layer, said cathode comprising a region of high conductivity semiconductor material and a low ohmic metal electrode disposed on the surface of said epitaxial layer between said anode and cathode such that said metal electrode overlaps the juncture between said region of high conductivity semiconductor material and said epitaxial layer, said anode being fabricated of a material different from the metal of said metal electrode and having poorer ohmic contact to said epitaxial layer than is exhibited by said metal electrode, said anode being operative to effect efficient extraction of current carriers from said epitaxial layer and consisting of a material selected from the group of materials consisting of highly conductive semiconductor material and material of the type forming a Schottky barrier with said epitaxial layer.
 2. A transferred electron device as claimed in claim 1 comprising a further electrode disposed on the surface of said epitaxial layer between said anode and said cathode, said further electrode comprising a further region of metal giving good ohmic contact with said epitaxial layer and a highly conductive semiconductor region adjacent a part of said further region of metal.
 3. A transferred electron device as claimed in claim 1 and comprising a plurality of further electrodes disposed in spaced relation to one another on the surface of said epitaxial layer between, and in spaced relation to, said anode and said cathode, each of said further electrodes comprising a region of high conductivity semiconductor material and a low ohmic metal layer disposed on the surface of said epitaxial layer in overlapping relation to the juncture between its associated region of high conductivity semiconductor material and said epitaxial layer. 